Replace hardcoded CRT shader with user-selectable file

[Eggbertx]

Instead of having a button to toggle the hardcoded CRT shader, it would be nice to have a button that shows a FileDialog, allowing the user to select their own shader, with the CRT (and maybe others) available as a sample.

[Ark2000]

Shaders layers like PPSSPP? good idea.

The crt shader was initially introduced as an easter egg, but if people need more shader stuff, I would like to add a screen shader manager.

[Eggbertx]

I don't think it's something that it necessarily needs, I just felt that if it's going to have shader loading in some form, it may as well be open ended and customizable by the user or developer (via PankuConsole.set_config`.